Agricultural formulations can be constructed to contain more than one active ingredient (a.i.) in a formulation. Examples of commercial herbicides that contain two active ingredients in a single formulation include alachlor/atrazine, bromacil/diuron, alachlor/ glyphosate, clomazone/trifluralin, cyanazine/atrazine, and 2,4-D/2,4-MCPA. Combining active ingredients is advantageous because it allows the formulated product to be efficacious over a broader range of target pests. However, combining multiple active ingredient materials in a single formulation can be problematical, and may not always be attainable. Pairing active ingredients is highly ingredient specific, and it must first be established that the actives are chemically stable in the presence of one another. It is not uncommon that when two complex organic molecules, such as pesticide active ingredients, are brought together that a chemical incompatibility exists that causes one or both of the active ingredients to undergo a change in its chemical identity. These changes can be due to chemical interactions occurring at reactive sites on either or both of the compounds involved, or reactions catalyzed by reactive impurities that may be present. The rate of these reactions, and therefore the extent of the chemical change observed, may be rapid or slow. In practice, the progress of the reaction, or lack thereof, in a formulated pesticide can be followed by conducting chemical assays of the product over a period of time, and recording the percent loss of active ingredient. Since chemical reactions are temperature dependent, one may even conduct assays at various storage temperatures and, with the aid of classical reaction-kinetics techniques, determine the rate constant of the reaction and the half-life of the actives. Such calculations are often used to estimate the shelf-life of a product.
When a chemical instability is discovered between two actives, various remedies to the problem may be considered. These generally fall into two categories namely, chemical modification and physical separation. Chemical modification approaches include: (i) selecting actives that are more chemically similar and therefore less likely to be antagonistic; (ii) conducting site-specific reactions during the manufacturing process to block or alter the offending site; or, if the loss in active ingredient is minor, (iii) adding compensatory active to the formulation during its manufacture. The physical separation options include any treatment that keeps the actives physically separated while still ostensibly presenting a single formulated system. These options include (i) microencapsulation; (ii) inclusion complexation; (iii) application of external water-soluble film coatings such as polyvinylpyrrolidone; (iv) employing compartmentalized packaging including water-soluble pouches; or, in the case of water dispersible granules, (v) employing a simple physical mix of each active granulated separately.
However, many of these approaches are undesirable as they usually entail extensive addition research, increased costs and other disadvantages. For example, altering the chemical structure of the active ingredient in order to neutralize a reactive site may change its biological effectiveness. Microencapsulation and complex formation are research intensive procedures with high raw material and processing costs. Physical mixes of two separately granulated dry flowables are subject to classification and segregation in the package if the granular size and density is not perfectly matched.
N-(4-fluorophenyl)-N-(1-methylethyl)-2[[5-(trifluoromethyl)-1,3,4-thiadiazo l-2-yl]oxy]acetamide, also known as "Fluthiamide" or "Flufenacet," and 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5-(4H)-one, also known as "Metribuzin," are two selective herbicides, from two distinctly different chemical families, which are highly effective against a range of broadleaf and grassy weeds found in corn and soybeans. Either of these active ingredients, when used by itself in a formulation, is stable during storage. However, when combined in a single formulation, as in the preferred dry form, these active ingredients are susceptible to chemical degradation. Therefore, there is a need to provide a dry agricultural composition, preferably a water-dispersible granule, of Fluthiamide and Metribuzin that is chemically stable and has commercially acceptable handling and biological performance properties.